1. Field of the Invention
This invention relates generally to memory cells, array structures of memory cells, and methods for programming the memory cells. More particularly, this invention relates to magnetic random access memory (MRAM) cells, array structures of MRAM cells, and methods for programming MRAM cells. Even more particularly, this invention relates to programming arrays of MRAM cells having segmented word lines.
2. Description of Related Art
As shown in FIGS. 1a and 1b, a memory array is generally formed of groups of MRAM cells 10 in columns and rows. Each MRAM cell 10 has an MTJ device 15 for retaining digital data as an orientation of the magnetic fields within the MTJ device 15. Each MTJ device 10 is formed of two layers of magnetic material 16 and 18 isolated from each other by a tunnel barrier 17. The free magnetic layer 18 is adjoined to the bit line 25. The bit line 25 conducts the bidirectional cell current Ic 35 such that the magnetic field developed by the bidirectional cell current Ic 35 in the bit line 25 and the row write cell current IR 40 in the row write line conductor 30 determine the magnetic orientation of the free magnetic layer 18. The direction of the bidirectional cell current Ic 35 determines the state of digital data within the MTJ device 15. The word line 30 conducts a row write cell current IR 40 in one direction. The magnetic orientation of the fixed magnetic layer 16 is determined during manufacturing of the MTJ device 15.
The fixed magnetic layer 16 is adjoined to a conductor 45 that is connected to the drain of an isolation transistor MISO 20. The source of the isolation transistor MISO 20 is connected to the ground reference point. The gate of the isolation transistor MISO 20 is connected to a read word line RWL
In the write operation of the MRAM cell 10, the direction of conduction of the bidirectional cell current Ic 35 determines the magnetic orientation of the free magnetic layer 18 and thus the digital data state retained by the MRAM cell 10. During the write process, the read word line RWL 50 deactivates the isolation transistor MISO 20 to prevent current conduction.
The read operation is illustrated in FIG. 1b. The read word line RWL 50 is set to a state to activate or turn on the isolation transistor MISO 20. The cell current Ic 55 is passed through the bit line 25, through the MTJ device 15, and the isolation transistor MISO 20 to the ground reference point. The magnetic orientation of the free magnetic layer 18 as compared to the magnetic orientation of the fixed magnetic layer 16 determine the resistance of the MTJ device 15. FIG. 2 shows the schematic diagram of the MRAM cell 10 with the MTJ device 15 and the isolation transistor MISO 20 serially connected. The read word line RWL 50 controls the activation and deactivation of the isolation transistor MISO 20. The bit line 25 is adjoined to the free magnetic layer 18 for reading the MRAM cell 10.
An MRAM array 100 of the prior art is illustrated in FIG. 3. The MRAM cells 105 are organized in rows and columns to form the MRAM array 100. Each MRAM memory cell 105 is structured and functions as described in FIGS. 1a and 1b. Each column of the MRAM memory cells 105 has a bit line 106a, 106b, . . . , 106n-1, 106n, 107a, 107b, . . . , 107n-1, 107n placed such that the bit line 106a, 106b, . . . , 106n-1, 106n, 107a, 107b, . . . , 107n-1, 107n is magnetically coupled to the free magnetic layer of each of the MRAM memory cells 105. Each bit line 106a, 106b, . . . , 106n-1, 106n, 107a, 107b, . . . , 107n-1, 107n is connected to a current source to receive the bidirectional cell current Ic.
Similarly, each row of the MRAM memory cells 105 has a segmented word line. As illustrated, a group of the MRAM cells are collected into separate blocks 110a and 110b. The rows of the MRAM cell block 110a have the word line segments 108a, 108b, . . . , 108m-1, 108m and the rows of the MRAM cell block 110b have the word line segments 109a, 109b, . . . , 109m-1, 109m. The word line segments 109a, 109b, . . . , 109n-1, 109n are placed such that each of the word line segments 108a, 108b, . . . , 108m-1, 108m and 109a, 109b, . . . , 109m-1, 109m is magnetically coupled to the free magnetic layer of each of the MRAM cells 105 on its associated row of MRAM cells.
One end of all of the word line segments 108a, 108b, . . . , 108m-1, 108m are connected to the source of the Block Select transistor 135a to select the MRAM cell block 110a. Each of the opposing ends of the word line segments 108a, 108b, . . . , 108m-1, 108m is connected to a drain of a Row Write Select transistor 120a, 120b, . . . , 120m-1, 120m. The drain of the Block Select transistor 135a is connected to the Word Line Current Source 145. The gate of the Block Select transistor 135a is connected to the Block Select Line 140a. The sources of each of the Row Write Select transistors 120a, 120b, . . . , 120m-1, 120m are connected to the current return line 150. Each of the gates of the Row Write Select transistors 120a, 120b, . . . , 120m-1, 120m is connected to a Row Write Select Line 115a, 115b, . . . , 115m-1, 115m. The Block Select Line 140a controls the activation and deactivation of the Block Select transistor 135a to control the flow of the Row Write Current IR from the current source 145 through a selected row segment of the MRAM memory cells 105 and a selected Row Write Select transistors 120a, 120b, . . . , 120m-1, 120m to the reference current return line 150. The Row Write Select Lines 115a, 115b, . . . , 115m-1, 115m control the activation and deactivation of the Row Write Select transistors 120a, 120b, . . . , 120m-1, 120m to steer the Row Write current from the Word Line Current source 145 through the selected word line segment 108a, 108b, . . . , 108m-1, 108m. 
One end of all of the word line segments 109a, 109b, . . . , 109m-1, 109m are connected to the source of the Block Select transistor 135b to select the MRAM cell block 110b. Each of the opposing ends of the word line segments 109a, 109b, . . . , 109m-1, 109m is connected to a drain of a Row Write Select transistor 125a, 125b, . . . , 125m-1, 125m. The drain of the Block Select transistor 135b is connected to the Word Line Current Source 145. The gate of the Block Select transistor 135b is connected to the Block Select Line 140b. The sources of each of the Row Write Select transistors 125a, 125b, . . . , 125m-1, 125m are connected to the current return line 150. Each of the gates of the Row Write Select transistors 125a, 125b, . . . , 125m-1, 125m is connected to a Row Write Select Line 115a, 115b, . . . , 115m-1, 115m. The Block Select Line 140b controls the activation and deactivation of the Block Select transistor 135b to control the flow of the Row Write Current IR from the current source 145 through a selected row segment of the MRAM memory cells 105 and a selected Row Write Select transistors 125a, 125b, . . . , 125m-1, 125m to the current return line 150. The Row Write Select Lines 115a, 115b, . . . , 115m-1, 115m control the activation and deactivation of the Row Write Select transistors 125a, 125b, 125m-1, 125m to steer the Row Write current from the Word Line Current Source 145 through the selected word line segment 109a, 109b, . . . , 109m-1, 109m. 
Each row of the MRAM memory cells 105 has a Read Word Line 130a, 130b, . . . , 130m-1, 130m connected to the gate of the isolation transistor of each of the MRAM memory cells 105. The Read Word Lines 130a, 130b, . . . , 130m-1, 130m control the activation and deactivation of the isolation transistors of each of the MRAM memory cells 105 with the selected row of the MRAM array 100 being activated during a read operation to conduct the read current from the associated bit line 106a, 106b, . . . , 106n-1, 106n, 107a, 107b, . . . , 107n-1, 107n through the MTJ device of the selected MRAM memory cells 105.
Writing one MRAM cell 105 or all the MRAM cells of a row segment of a block 110a or 110b of the MRAM memory cells 105 is shown in the plot of FIG. 4. The Block Select Line 140a or 140b is activated to turn on the selected Block Select transistor 135a or 135b for the chosen MRAM cell block 110a or 110b. The Row Write Select Line 115a, 115b, . . . , 115m-1, 115m for the selected row segment of the MRAM cell block 110a or 110b is activated to turn on the Row Write Select transistors 120a, 120b, . . . , 120m-1, 120m and 125a, 125b, 125m-1, 125m to steer the Row Write current IR through the selected word line segment 108a, 108b, . . . , 108m-1, 108m or 109a, 109b, . . . , 109m-1, 109m at the time τ1. The bidirectional cell current Ic is applied at the time τ2 to the selected block of MRAM cells through the appropriate bit lines 106a, 106b, . . . , 106n-1, 106n, 107a, 107b, . . . , 107n-1, 107n. At the time τ3, the Block Select Line 140a or 140b is deactivated to turn off the selected Block Select transistor 135a or 135b for the chosen MRAM cell block 110a or 110b and the programming of the selected row segment of the selected MRAM cell block 110a or 110b is completed by the termination of the positive or negative bidirectional cell current +IC or −IC at the time τ4.
The bidirectional cell current Ic is either a positive +IC or negative −IC current dependent on the state of the digital data to be programmed to the selected MRAM cells 105.
“High Speed (10-20 ns) Non-Volatile MRAM with Folded Storage Elements,” Ranmuthu, et al., IEEE Transactions on Magnetics, Sep. 1992, Vol. 28, Issue 5, pp. 2359-2361 describes an MRAM chip has been designed using 250 Ω folded memory cells, two-turn word lines, and a high-speed differential sensing scheme.
“Optimizing Write Current and Power Dissipation in MRAMs By Using an Asteroid Curve,” Miyatake, et al., IEEE Transactions on Magnetics, May 2004, Vol. 40, Issue 3, pp. 1723-1731 describes the analytical expressions of minimum electric current and power dissipation, and bit line and word line currents that produce them, for writing data into magnetic tunnel junction (MTJ) magneto resistive random access memory (MRAM) cells are derived with the assumption that an asteroid curve can be applied to all MTJs in a memory cell array. The expressions contain word length, that is, the number of bits per word, and parasitic resistances of the write word line and bit line (which are important design parameters of memory cell arrays) and distances between the write currents and the free magnetic layer for data storage (which are important structural parameters of MTJ cells). They provide quantitative MRAM design guidelines and help to understand current and power behavior.
U.S. Pat. No. 6,490,217 (DeBrosse, et al.) teaches an MRAM memory device that has a multiple segmented groups. Each segmented group includes a number of memory cells operatively coupled to a corresponding segmented word line. Each segmented word line is disposed in relation to the plurality of corresponding memory cells such that the destabilizing current passing through the segmented word line destabilizes the corresponding memory cells for writing.
U.S. Pat. No. 6,584,006 (Viehmann) provides a segmented MRAM bit line and word line architecture. Switch circuits are coupled to and located along the adjacent bit lines resulting in the array being divided into segments along the adjacent bit lines. The segmenting of the bit lines and word lines shortened the programming current path that results in decreased resistance across the device.
U.S. Pat. No. 6,816,405 (Lu, et al.) describes a segmented word line architecture for cross point MRAM arrays. The MRAM array magnetic memory cells is arranged in rows coupled to local word lines for assisting in writing a logical state of the at least one memory cell. The MRAM array further has global word lines connected to at least one of the plurality of local word lines. The global word lines are substantially isolated from the memory cells. Write circuits are operatively coupled to the global word lines. The write circuits are configurable as a current source and/or a current sink for supplying and/or returning, respectively, at least a portion of a write current for assisting in writing one or more memory cells. The write circuits are configured to selectively distribute the write current across at least a plurality of global word lines so that stray magnetic field interaction between selected memory cells and half-selected and/or unselected memory cells is reduced.
U.S. Pat. No. 6,870,759 (Tsang) and United States Patent Application 2004/0165424 (Tsang) illustrate an MRAM array with segmented magnetic word lines. Each of the segment word line is coupled with the global word line(s) such that each segment is separately selectable. Each segment is coupled to a portion of the magnetic storage cells.
United States Patent Application 2004/0190360 (Scheuerlein) describes a word line arrangement having multi-layer word line segments for three-dimensional memory array. The three-dimensional (3D) passive element memory cell array provides short word lines while still maintaining a small support circuit area for efficiency. Short, low resistance word line segments on two or more word line layers are connected together in parallel to form a given word line without use of segment switch devices between the word line segments. A shared vertical connection preferably connects the word line segments together and connects to a word line driver circuit disposed generally below the array near the word line. Each word line driver circuit preferably couples its word line either to an associated one of a plurality of selected bias lines or to an unselected bias line associated with the driver circuit, which selected bias lines are themselves decoded to provide for an efficient multi-headed word line decoder.